Nuts & Volts - May 2004

Near Space

KD4STH- 11>APT310,WIDE3-3:/143130h4313.72N/11721.12W>272/029/A=111840

Example 1

The callsign remains fixed for the entire mission of the NS craft (if not for every one of its missions). The - 11 after the callsign is unique to packet radio and is called an SSID. Packet stations use SSIDs so that a licensed amateur radio operator can own several packet stations that run simultaneously. The SSID is used to identify the station transmitting the packets. In the example above, the - 11 SSID is used to identify the transmitting station as a balloon.

The next field indicates which version of APRS is being used by the TNC (Tiny Trak 3, in this case). This field stays constant for the Tiny Trak 3.

The routing information — in this example, “WIDE3-3” — indicates how many “bounces” a packet signal is allowed. Packet signals can be digi-peated. This occurs when a packet station that is set up to do digi-peating receives the posit report from your NS craft and retransmits it. The “- 3” indicates that the posit report is allowed to be digi-peated a maximum of three times. The proper number of repeats appropriate for your module is determined by the standards used by packet radio operators in your area. Contact your local APRS user community before entering your WIDE setting. Both the callsign and routing information are programmed into the Tiny Trak 3 before launch.

The time stamp in the posit is derived from the GPS receiver and is given as time — UTC (think Greenwich Mean Time). The time stamp format is HHMMSS, with the hours given in 24-hour time. There are no commas between the hours, minutes, and seconds. The field ends with “h.”

Latitude and longitude are given as degrees and minutes. Notice that there are no delimiters between the degrees and minutes. The minutes

MAY 2004

of latitude contain a decimal point. The field ends in the letter “N,” indicating that the field is latitude north; the “W” in the latitude field indicates west. There is a slant separating the latitude and longitude fields.

The next two fields are pulled from the GPRMC sentence.

The first is the heading of the GPS receiver (the direction that the GPS is traveling). Since the GPS is attached to the NS craft and its balloon — which is prey to the ambient winds — this field is also the direction of the wind. The heading is measured by true north, not magnetic north.

The second GPRMC field is the speed at which the GPS receiver is traveling. This is also the wind speed. The unit of speed from a

GPS receiver is given in knots. A slash is used to separate the heading and speed fields.

The last field is altitude, measured in feet. The Tiny Trak 3 converts the altitude units from the GPS receiver from

meters into feet. The altitude field is indicated by the “A=” preceding the altitude.

This is all there is to the Tiny Trak posit report. If you’d like more information on the format of posit reports, consult the APRS Protocol Reference site ( http://web.usna.navy.mil/ _~bruninga/ aprs.html).

Processing Tiny Trak 3 Posits

Now that you know the format of the posits that you have received from the NS craft, let’s discuss how to process the data after a mission; this is called post-processing. Did you think the mission was over once